The use of local orbitals in multireference calculations

CAS-SCF-type algorithms based on molecular orbitals that preserve their physical nature during the iterative process have been proposed recently by our groups. Our approach is based on the iterative partial diagonalization of the one-body reduced density matrix. If localized guess orbitals are used, the locality property is kept by the final orbitals. The use of local orbitals in multiference calculations has several advantages. It can be used to reduce the number of active orbitals in CAS-SCF calculations on large systems, and in general to have a better control on the physical nature of the active space. The reduction from a complete to a selected reference space is also possible in the case of configuration interaction calculations. The technique is illustrated through applications to the description of bond breaking and n --> pi* excited states in conjugated systems. The efficiency of selection of local excitations is shown on a magnetic complex.